<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <script src="https://cdn.staticfile.org/jquery/1.10.2/jquery.min.js"></script>
    <script>
        $(function () {
            $("input").focus(function () {
                $("span").css("display","inline");
            });
            $("input").blur(function(){
                $("span").fadeOut(2000);
            });
        })
    </script>
    <style>
        span {
            display: none;
        }
    </style>
</head>
<body>
    <div id="app">
        <input type="text"/>
        <span>请输入你的电话号码</span>
    </div>
</body>
</html>